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Chemistry

Polymers – Plastics

Polymers are giant molecules which are formed by joining together a large number of much smaller molecules thus, forming a long chain. The smaller molecule, which is the starting material, is known as monomer (meaning one unit). The molecular size of a given polymer is not fixed.

There are two types of polymers: synthetic and natural.

Synthetic polymers:

are derived from petroleum oil, and made by scientists and engineers. Examples of synthetic polymers include nylon, polyethylene, polyester, Teflon, and epoxy.

Natural polymers:

occur in nature and can be extracted. They are often water-based. Examples of naturally occurring polymers are silk, wool, DNA, cellulose and proteins. Rubber and many resins are also natural polymers found in plants.Polymerization is any process in which relatively small molecules, called monomers, combine chemically to produce a very large chain-like or network molecule, called a polymer. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. Usually at least 100 monomer molecules must be combined to make a product that has certain unique physical properties—such as elasticity, high tensile strength, or the ability to form fibres. The formation of stable covalent chemical bonds between the monomers sets polymerization apart from other processes, such as crystallization, in which large numbers of molecules aggregate under the influence of weak intermolecular forces.

Two classes of polymerization usually are condensation and addition polymerization.

 Condensation polymerization is the process whereby two or more monomers link together to form the polymer with the elimination of a small molecule. In condensation polymerization, each step of the process is accompanied by formation of a molecule of some simple compound, often water. Two most important condensation polymers are nylon and terylene.

Addition polymerization is the process whereby two or more of the same monomers link together to form the polymer without elimination of any small molecules. In addition polymerization, monomers react to form a polymer without the formation of by-products. Addition polymerizations usually are carried out in the presence of catalysts, which in certain cases exert control over structural details that have important effects on the properties of the polymer. Addition polymers include poly(ethene), poly(chloroethene), perspex, etc.

Other Terms

Linear polymers: which are composed of chain-like molecules, may be viscous liquids or solids with varying degrees of crystallinity. A number of them can be dissolved in certain liquids, and they soften or melt upon heating.

Cross-linked polymers: in which the molecular structure is a network, are thermosetting resins (i.e., they form under the influence of heat but, once formed, do not melt or soften upon reheating) that do not dissolve in solvents.

Note: Both linear and cross-linked polymers can be made by either addition or condensation polymerization.

Uses of Polymer:

Plastics

Plastics are natural/synthetic materials. They are produced by chemically modifying natural substances or are synthesized from inorganic and organic raw materials. They are giant molecules which are products of polymerization of simple unsaturated compounds like ethene, propene or substituted unsaturated compounds like styrene (phenylethene) and vinyl chloride chloroethene. Plastics also include synthetic fibres like nylon and terylene. Plastics are either:

Thermoplastic: This can soften upon heating and return to their original form. They are easily molded and extruded into films, fibers and packaging. Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

Thermoset or thermosetting plastics: Once cooled and hardened, these plastics retain their shapes and cannot return to their original form. They are hard and durable. Thermosets can be used for auto parts, aircraft parts and tires. Examples include polyurethanes, polyesters, epoxy resins and phenolic resins.

Polyethylene, LDPE and HDPE: The most common polymer in plastics is polyethylene, which is made from ethylene monomers (CH2=CH2). The first polyethylene was made in 1934. Today, we call it low-density polyethylene (LDPE) because it will float in a mixture of alcohol and water. In LDPE, the polymer strands are entangled and loosely organized, so it’s soft and flexible. It was first used to insulate electrical wires, but today it’s used in films, wraps, bottles, disposable gloves and garbage bags.

In the 1950s, Karl Ziegler polymerized ethylene in the presence of various metals. The resulting polyethylene polymer was composed of mostly linear polymers. This linear form produced tighter, denser, more organized structures and is now called high-density polyethylene (HDPE). HDPE is a harder plastic with a higher melting point than LDPE, and it sinks in an alcohol-water mixture. HDPE was first introduced in the hula hoop, but today it’s mostly used in containers.

Polyvinyl Chloride (PVC): PVC is a thermoplastic that is formed when vinyl chloride (CH2=CH-Cl) polymerizes. When made, it’s brittle, so manufacturers add a plasticizer liquid to make it soft and moldable. PVC is commonly used for pipes and plumbing because it’s durable, can’t be corroded and is cheaper than metal pipes. Over long periods of time, however, the plasticizer may leach out of it, rendering it brittle and breakable.

Polystyrene (Styrofoam): Polystyrene is formed by styrene molecules. The double bond between the CH2 and CH parts of the molecule rearranges to form a bond with adjacent styrene molecules, thereby producing polystyrene. It can form a hard impact-resistant plastic for furniture, cabinets (for computer monitors and TVs), glasses and utensils. When polystyrene is heated and air blown through the mixture, it forms Styrofoam. Styrofoam is lightweight, moldable and an excellent insulator.

Polypropylene (PP): In 1953, Karl Ziegler and Giulio Natta, working independently, prepared polypropylene from propylene monomers (CH2=CHCH3) and received the Nobel Prize in Chemistry in 1963. The various forms of polypropylene have different melting points and hardnesses. Polypropylene is used in car trim, battery cases, bottles, tubes, filaments and bags.

Nylon: Nylon fibre is obtained by heating hexane dioc acid (adipic acid) with hexane 1,6 diamine. Nylon was originally developed as a textile but is available in many forms with vastly different properties. Engineering nylon grades are easy to machine with good resistance to biological attack. Unfortunately nylons can absorb moisture from the atmosphere and can degrade in strong sunlight (they are unstable in ultraviolet light) unless a stabilizing chemical is added at the initial manufacture of the plastic. Nylons are easy to mould. Nylons also have a natural ‘oily’ surface that can act as a natural lubricant. Nylons are used for everything from clothes through to gears and bearings. It is also slippery and can be used to make washers, spacers and bushes.

EpoxyResin: is a thermosetting polymer formed from reaction of an epoxide resin with polyamine hardener. The resin consists of monomers or short chain polymers with an epoxide group at either end. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol A. Epoxy resin has various uses such as; the resins that are cured through exposure with ultra violet light are normally used in optoelectronics, fibre optic and dentistry. Industrial tooling applications normally use resin to make laminates, fixtures, castings and moulds. In the electronic world, epoxy resin is used to make transformer, insulator, switch gear and generators.

Terylene: This is a polyester that is also known as Dacron in the U.S.A. It is formed by the condensation of benzene- 1,4- dicarboxylic acid (terephthalic acid) and ethane- 1,2- diol (ethylene glycol), using an acid catalyst.

Terylene is another synthetic fibre which is mainly used in the manufacture of synthetic textiles. Terylene is mostly used for clothing, ropes, sheets, sails and many others.

Natural Rubber

Rubber is obtained from the rubber tree, Hevea brasiliensis. When the bark of the tree is cut, a thick white liquid called latex oozes out. if the latex is collected and heated, it changes into an elastic solid called rubber. This rubber is of little use because it is soft and sticky. Chemically, it consists of 2-methyl buta-1,3-diene monomers known previously as isoprene. When the monomers polymerize, they form a long polymeric chains which have only a limited number of cross-links between them. By adding sulphur and heating, the soft rubber becomes hard.

Vulcanization is chemical process by which the physical properties of natural or synthetic rubber are improved. In its simplest form, vulcanization is brought about by heating rubber with sulfur. Finished rubber has higher tensile strength and resistance to swelling and abrasion, and is elastic over a greater range of temperatures.

Synthetic Rubber

1,3-Butadiene is an important industrial chemical used as a monomer in the production of synthetic rubber. Buta-1,3-diene co-polymerize with phenylethene to form the tyre of synthetic rubber known as bunas or styrene butadiene rubber (SBR).

ASSESSMENT (POST ANSWERS BELOW USING THE BOX)

  1. ……… are giant molecules which are formed by joining together a large number of much smaller molecules thus, forming a long chain.
  2. Explain Epoxyresin.
  3. What do you understand by Polystyrene?

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